Capital Volume 1, Results of the Immediate Process of Production. Part 3: The Einzelne Seiten

When the labour of the worker begins it has already ceased to belong to her: labour-power is sold before it is used. To put it another way, the use-value of labour-power passes into the hands of the buyer after it is sold as an exchange-value. Its exchange-value is determined before it is sold, according to the labour time previously expended to produce it. ‘The exchange-value of this commodity existed […] before its sale, while its use-value consists only in the subsequent expression of its power.’ But the money laid out by the buyer functions as means of payment, not means of purchase, for, even though labour-power is sold before it is used, it is routinely only paid for afterwards. The worker, by advancing her labour-power to the capitalist, effectively allows her credit.